Treatment planning study of cyberKnife prostate SBRT (stereotactic body radiation therapy) using CT-based vs MRI-based prostate volumes

This study has been conducted for the purpose of investigating the systematic dose reduction of rectum and neurovascular bundles (NVBs) during treatment planning of the CyberKnifeTM prostate SBRT using CT-Based volumes versus MRI-based volumes. Three prostate cancer patients were Planned for the CyberKnifeTM prostate SBRT and they underwent computed tomography (CT) and magnetic resonance imaging (MRI) preplanning exams. The patients were positioned during both exams using an immobilizing device. A radiation oncologist and a radiologist delineated the prostate gland, intra-prostatic and peri-prostatic structures, and pelvic organs of interest in both CT and MRI images. The CT and MRI images were fused based on fuducial markers to accurately align the prostate. Radiation Therapy Oncology protocol RTOG 0938 was followed to meet the target volume (prostate plus margin) dose coverage requirement, and dose-volume constraints for organs at risk, including rectum, bladder, femoral heads, penile bulb, urethra, skin and NVBs. Radiation dose volume parameters were recorded for both volumes and compared. The preliminary result shows that the CT-based volumes were generally larger than MRI-based volumes of the prostate. Therefore, the CT-based volumes resulted in less accurate treatment planning and dose delivery to radiosensitive structures